I’ve purchased keys from vendors on Ebay & Amazon only to have MS rejected the key after a period of time.
Amazon too? Damn.
They are often the same vendors. I generally trust the eBay seller feedback more than the Amazon. So I guess that I need to count my lucky stars, because I have never had any of the product keys that have I purchased rejected.
But you are scaring me so I went ahead and started up a few virtual machines with some of my older licenses to try and verify. It could still be just a matter of time I guess... This includes Windows XP, Windows 7, Windows 10, and Microsoft Office licenses. I do not buy the absolutely cheapest licenses listed, and I only buy from sellers with large amounts of good feedback who have been around for awhile. These have all mostly been used in virtual machines.
OK, I found that one of my Windows 7 virtual machines is now reporting that it is “not genuine”. I guess that is $5 that I will never get back.